[client] x11: change to a more visible "dot" cursor

This change alters the small square dot cursor to a more visible 16x16
cursor for X11. A new option `spice:largeDotCursor` can be set to use an
alternative 32x32 cursor for the vision impaired.

client/include/interface/displayserver.h

@@ -90,6 +90,7 @@ typedef struct LG_DSInitParams
   bool resizable;
   bool borderless;
   bool maximize;
+  bool largeCursorDot;
 
   // if true the renderer requires an OpenGL context
   bool opengl;